A SALM buoy installation can include a buoyant riser such as a tall column extending from the sea floor to the sea surface, and having a swivel unit, or product distribution unit (PDU) therealong. A mooring line or hawser can extend from a ship to the upper portion of the column which lies above the PDU, while underwater hoses can lead from the PDU to the ship to carry oil or other cargo between them. The PDU can rotate about the vertical axis of the column to follow a drifting ship. The PDU is subject to wear, and occasionally must be replaced. This has typically necessitated the removal of the entire upper column portion lying above the PDU, and since this structure is massive and highly buoyant it requires considerable time and expense to replace the PDU. Although divers can reach the PDU which is typically located a moderate distance under water, and could even remove it from the rest of the column structure, the removal of the PDU which lies coaxial with the column normally has left the upper buoyancy structure free to move about, and its large size and buoyancy makes it difficult to handle. A SALM buoy installation which enabled the replacement of a PDU located coaxially with and along a riser such as a column, without requiring the freeing of the upper buoyancy structure portion of the column, would facilitate the replacement of damaged PDUs. In many situations, it would be even more desirable if cargo transfer could continue during the period when a malfunctioning PDU of the above-described type was being replaced.